1. Field of the Invention
The present invention generally relates to a multilayer electronic substrate which is employed in an electronic appliance. More specifically, the present invention is directed to a multilayer electronic substrate, a manufacturing method of the multilayer electronic substrate, and a use thereof, capable of easily performing a trimming step, while protecting an under-layer conductor when a resistor is trimmed, and also, capable of avoiding that a resistor is shortcircuited to an under-layer conductor layer by an electric conductive material of such a resistor which is melted during the trimming step.
2. Description of the Related Art
In a multilayer electronic substrate in which a resistor is trimmed so as to adjust an electric characteristic of a circuit, for example, a laser beam is irradiated to a resistor mounted on the multilayer electronic board so as to adjust a resistance value. When laser beams are irradiated to this resistor, if power of a laser beam is low, then a resistor element cannot be sufficiently cut, and thus high reliability cannot be obtained. Also, if power of a laser beam is excessively high, then there are some risks that an insulator is cut, and furthermore, a conductor (the ground) of an under layer of this insulator is cut. As explained above, the cutting depth of the insulator is adjusted by controlling the output power of the laser beams. Since the thickness of the insulator such as the multilayer electronic substrate is thin, it is practically difficult that the cutting depth is adjusted in the fine mode by controlling the power of the laser beams.
As a result, in a conventional trimming step, a heat-resisting surface layer portion is provided on a surface of an insulating layer of a position (under layer) which corresponds to such a portion of a multilayer electronic substrate, to which a laser beam is irradiated, so that it is possible to avoid that the laser beam is irradiated to a deep depth position of this multilayer electronic substrate. Thus, an under-layer conductor can be protected, and the trimming step can be carried out in an easy manner (refer to, for example, JP-A-6-77665 (Page 4, FIG. 1)).
In the trimming step described in, however, the following problems occur. That is, since the heat-resisting surface layer portion of the multilayer electronic substrate is provided, the thickness and the weight of the circuit are increased. Also, the material cost and the processing step number are increased.